1. Field of the Invention
This invention concerns a multi-functional solid state, electrical retrofit packaging arrangement for converting any medium voltage type E-2 motor controller, existing or new build, vacuum or air-break, into a solid state soft start controller.
2. Description of the Prior Art
Pioneering research by English physicist Michael Faraday led to the discovery of electromagnetic induction in 1830. Using a ring of soft iron, Faraday wound separate coils of wire on opposite ends of the ring. He connected one of the wire coils to a battery, causing a flow of electric current through the wire coil, magnetizing the iron ring and inducing current into the second wire coil. Current flow into the second coil was controlled with a manually operated switch which opened and closed the circuit, starting and stopping the flow of current.
In 1832, Joseph Henry, an American physicist engaged in experimental work similar to Faraday's by publishing a technical paper describing his own observations in the new found physics of electromagnetic induction. Henry described experiments in which large, bright electric arcs were produced when disconnecting a current carrying electrical cable from a battery. He noted that the wire and the battery terminal (current source) caused the current to continue its flow between the two connections while being pulled apart. The same phenomenon could be observed in a device such as a manual switch. Henry referred to this electric arcing phenomenon as "self-induction".
Development of the world's first electric generators and motors evolved out of the pioneering research by Faraday and Henry. Commercial production of the first prototype electric generators began in 1850 and motors in 1870. These early machines were powered by direct current (DC). In 1888 the first successful electrical system in America utilizing alternating current (AC) was developed by Nikoli Tesla. The wide use of AC power was furthered by the scientific efforts of early electrical engineers such as George Westinghouse and Charles Steinmetz. Although DC motors and devices continue in use, AC machines are the most prominent devices of choice. From the beginning serious technical problems were encountered in the starting and stopping of AC & DC rotating electric machines.
Research and engineering in the starting and stopping of AC & DC rotating electrical machines has evolved, out of necessity, into a highly specialized area of electrical engineering. Electrical devices for opening and closing motor circuits have evolved from the simplistic manually operated air-break switches used by Faraday and Henry into highly sophisticated vacuum contactors capable of handling enormous motor horsepower loads at voltages ranging up through 15 Kv.
Until the introduction of vacuum technology about 25 years ago, variations of air-break switches, breakers, magnetic contactors and combinations of these devices in numerous configurations were the only practical devices available to start and stop AC & DC rotating electrical machines. Vacuum technology offers an alternative to air-break devices by confining the arcing phenomenon observed by Joseph Henry to the interior of a sealed vacuum container. Vacuum technology offered so many inherent advantages over the old air-break method that it quickly became the technology of choice for controlling AC & DC rotating electrical machines of all sizes. Today, vacuum contactors are the predominant devices of choice in controlling the start-stop application of medium voltage motors (2.4 Kv-15 Kv) of all types.
About 40 years ago electrical researchers began experimenting with solid state motor starting techniques. This research explored various methodologies for starting AC and DC rotating electrical machines. A solid state motor starter uses electronic circuitry in place of the traditional contactors or switches to start and stop a motor. Dependable low voltage solid state starters for small motor loads became commercially available about 20 years ago and today are available for a wide range of motor loads and higher voltages. Inherent characteristics of the solid state starter are its low starting current requirements in bringing a large motor load up to full speed and its controlled soft stop capability when taken off line. This is especially advantageous in applications involving large medium voltage motors. These types of controllers are referred to commercially as a "solid state soft start". High cost, sophisticated circuitry and technical limitations have traditionally restricted use of these devices to specialized applications. Solid state soft start controllers for large medium voltage motor applications are domestically manufactured by only a few specialty companies.
Within the solid state family of devices, controllers known as "variable speed drives" were an early product of solid state motor controller research. In addition to starting and stopping duty, these devices also have speed control capability of the rotating electrical machine(s) during normal operation and start-stop cycles. These are hybrid devices using a sophisticated combination of solid state electronics, magnetic or vacuum contactors and other components configured into a "control system". They are available from numerous foreign and domestic manufacturers for a full range of motor applications from low voltage through medium voltage. High cost and sophisticated technical nature of these systems restrict their use to highly specialized applications.
Today, there are three commonly used devices utilized for the control of a medium voltage motor: (1) vacuum controller/full voltage--"Across the Line", (2) solid state soft start, and (3) variable speed drive. A variable speed drive package is expensive and restricted to a drive oriented application requiring the specific characteristics and features offered by a drive controller. A solid state soft start has inherent characteristics which are superior to the full voltage--"across the line" vacuum contactor in controlling large medium voltage motors, but in the past were cost prohibitive.
Motor starter assemblies exist which are packaged in a roll out or drawout frame for being plugged into or unplugged from a power distribution panel. One such assembly is manufactured by the General Electric Company and is described in its brochure titled Limitamp.RTM. Medium Voltage Motor Control for motors rated between 2400-7200 volts.
One starter assembly, known as the CR7160 ampere air-break contactor, includes a motor starter assembly which is mounted on a frame and includes upper and lower disconnect plugs for each of the three leads of a power bus at its forward end. The assembly is arranged to be rolled into a power distribution panel which has a space to receive the starter assembly. At the back end of the space in the starter assembly are upper and lower plugs coupled to leads for a main power bus and for a bus to a motor. The plugs of the assembly are arranged such that when the assembly is inserted into the space of the power distribution panel, the motor starter assembly plugs mate with the plugs in the panel. The assembly includes an electrical path between the two plugs which include a fuse and a contactor.
The prior art Limitamp.RTM. motor starter assemblies include either an air-break contactor or a newer vacuum break contactor. Such contactors close the electrical path of the motor starter by control of a DC operating coil designed to be used with a holding impedance that is inserted after the contactor is fully closed to limit coil current. The contactor coil is designed for use on 120 Volts (AC) rectified or 125 Volts DC control source. A fuse is mounted in the motor starter assembly. The Limitamp.RTM. motor starter assembly can be pivoted to an open position after it is drawn out of the power distribution panel. The pivoting of the top part of the frame of the motor starter with respect to the bottom part of the frame exposes all integral parts for easy inspection and maintenance.
A great number of motor starter units of the type described above are in operation, not only in the U.S.A., but in other parts of the world as well. Such existing motor starter assemblies, while functioning well as on-off switches, cannot achieve the advantages of a soft start motor starter circuit which can apply a.c. current gradually to motor windings from a power bus until the motor reaches full speed or vice versa.
A solid state soft start circuit for connecting an a.c. motor to a high voltage power line is superior to a "full voltage--across the line" air or vacuum contactor in controlling large medium voltage motors, but in the past were cost prohibitive.
A great need exists to convert existing motor starter units of the on-off variety into modem soft-start motor starter devices, while providing such converted or improved motor starter units to be placed within the exterior profile dimensions of existing units.